1. Field of the Invention
The present invention relates to an apparatus for a frequency synthesizer, and more specifically a frequency synthesizer with a high resolution digitally controlled oscillator.
2. Description of Related Art
A digitally controlled oscillator (DCO) is a type of variable frequency oscillator which has a frequency of oscillation that is a function of a digital input signal. Other types of common oscillators include voltage controlled oscillators (VCO), and current controlled oscillators (ICO). These oscillators however are very sensitive to 1/f noise, and input voltage/current noise. The input voltages and currents are susceptible to interference noise and drift. These oscillators also are very susceptible to frequency drift. DCOs have the possible advantage of exhibiting much better noise immunity, can easily maintain a desired frequency, require less redesign effort due to scaling, may have lower power dissipation, and are easy to implement in low cost CMOS processes. However most DCO configurations have a limited resolution due to matching and size limitations (6-10 bits typ.). One of the most common implementations of a DCO is in a phase lock loop PLL. In a PLL the DCO must be capable of being adjusted in small enough increments so as to result in quantization related frequency and phase jitter which are below predetermined limits. Furthermore, the delay element of the present invention has highly reduced sensitivity to 1/f noise. It is achieved by using a circuit with reduced sensitivity because control is digital.
Another major problem in PLL design is clock jitter. Clock jitter can be defined as the deviations in a clock""s output transition from their ideal positions, whether it be leading or lagging. Causes of clock jitter can include power supply noise, random thermal noise, mechanical vibration, and a dead-band within the phase detector that does not detect small phase errors. DCO""s have a disadvantage of not being able to generate a continuous range of frequencies, but instead produce a finite number of discrete frequencies. This makes it desirable to have small quantization steps to reduce jitter as much as possible. A specific embodiment of the invention utilizes its design in a phase lock loop for digitizing RGB signals. In such an application a further major cause of jitter is the inputted reference signal to the PLL. In typical analog PLLs for this application minimizing the jitter from the reference signal is a function of the PLL bandwidth. However by narrowing the bandwidth of the PLL, the PLL""s ability to correct long term jitter becomes more limited.
Therefore a need exists for a frequency synthesizer with high immunity to jitter and noise, and can be manufactured in a low cost CMOS process.
In one preferred embodiment of the invention a quantizer with a high resolution input is connected to a low resolution digitally controlled oscillator (DCO).
It is a further object of the present invention to provide such a DCO that operates with low levels of jitter.
Another object of the invention is to provide a DCO controlled by a high resolution input.
In order to achieve the above objectives, according to an aspect of the present invention, a high resolution DCO frequency synthesizer is provided that allows for low-jitter and improved noise immunity. The frequency synthesizer of the invention in its simplest form consists of a noise-shaping quantizer with high resolution input, and a digitally controlled oscillator with low resolution input. The quantizer spectrally shapes the quantizer error pushing most of the error into higher frequencies and therefore lowering error in the operating frequency range. The DCO acts naturally as a lowpass filter removing much of the high frequency error.
These and other features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the drawings, and to the accompanying descriptive matter, in which there is described exemplary embodiments of the invention.